Air pollution in form of particulates is a major concern in the world today and the amount of particulates in the air is increasing in many places. Some particulates occur naturally in the air, originating e.g. from volcanos, dust storms and forest/grassland fires. Some particulates are caused by human activities, such as burning of fossil fuel and industrial processes. Increased levels of particulates in the air are linked to several health hazards.
The levels of particulates in the air may be particularly high in large cities with many inhabitants, vehicles and industries. Legislation in some countries relates both to the emission allowed for a certain type of pollution and for the ambient concentration of particulates. For example, the European Union has established the European emission standards, which include limits for particulates in the air. Particles with a diameter of 10 micrometers or less are often referred to as PM10 and particles with a diameter of 2.5 micrometers or less are often referred to as PM2.5.
In order to reduce the levels of particulates in the air, some countries regulate the amount of particulates allowed in exhaust gas emitted from vehicles. In order to pass the regulations, vehicles equipped with combustion engines must often be provided with a particulate filter as a part of a vehicle exhaust gas treatment system. The particulate filter may remove most of the particulates, such as soot, when the exhaust air from the combustion engine passes the particulate filter. Some particulate filters are single-use filters, intended for disposal and replacement once full of accumulated soot. Some particulate filters are designed to burn off the accumulated particulates, e.g. passively through use of a catalyst, or actively by active means such as a fuel burner that heats the particulate filter to particulate combustion temperatures. In some vehicles the exhaust gas temperature may be temporarily increased such that particulates in the particulate filter are combusted. Such particulate filters may be referred to as regenerable particle filters. A particulate filter may be e.g., a diesel particulate filter (DPF) or a petrol/gasoline particulate filter (GPF).
Some particulate filters of today are very efficient and may collect a large amount of soot and small particulates in air passing the filter. However, it is still desirable to further reduce the amount of particulates in the air.